Abstract:The Amazonian fish species Plagioscion squamosissimus (Sciaenidae) and Cichla monoculus (Cichlidae), have been widely introduced into different reservoirs in Brazil, and have caused many negative impacts on local fish fauna. The aim of this study was to evaluate the population structure (abundance, length structure, length-weight relationship, sex ratio, and length at first maturity) and the reprodutive period of these two species in the Santa Cruz Reservoir (built in 2002), located in the Brazilian semiarid region, for their adequate management and local species conservation policies. Specimens were collected quarterly in eight sites from February 2010 to November 2013 using gillnets (12 to 70 mm mesh between adjacent knots). The specimens captured were counted and the following biometric and biological data were analysed: standard length, total weight, and reproductive data, such as, sex, weight and gonadal maturity stage. The species abundances were estimated by CPUE and expressed as the number of individuals per gill net area x gill net exposition time (m 2 x h); length frequency histograms were built with intervals of 5 cm. The length-weight parameters were estimated with a linear regression after a logarithmic transformation of the data. With the reproductive data we estimated sex ratio, reproduction period and length at first maturity (L 50 ). We captured a total of 1 071 specimens of P. squamosissimus and 156 specimens of C. monoculus. Both species showed higher abundances in 2010, 0.004306 m 2 x h and 0.00022 m 2 x h, respectively, but this parameter decreased from 2010 to 2013. Standard length ranged between 6.4 and 46.2 cm for P. squamosissimus (20.025.0 cm was the most frequent class), and 7.0 and 38.7 cm for C. monoculus (10.0-15.0 cm was the most frequent class). The length-weight relationships were described by the following equations: log 10 Wt = -1.8349+3.0899log 10 Lp and R 2 = 0.9795 for P. squamosissimus, and log 10 Wt = -1.7944+3.0885log 10 Lp and Wt = 0.0160 and R 2 = 0.9929 for C. monoculus; both species exhibited positive allometric growth. The sex ratio for both species differed from 1:1, with a predominance of females for P. squamosissimus and males for C. monoculus. The Lp 50 was estimated to be 15.90 cm for P. squamosissimus and 15.65 cm for C. monoculus, and the reproductive data indicated that both species reproduced throughout the year. We concluded that although the population of both species reduced their abundance over the study period, P. squamosissimus and C. monoculus are established and structured populations in the Santa Cruz Reservoir with individuals growing satisfactorily and reproducing in all seasons, without a well-defined reproductive peak. We suggest that the managers may allow the artisanal and/or sport fisheries of these species. Rev. Biol. Trop. 63 (3): 727-739. Epub 2015 September 01.Key words: Cichla monoculus, Plagioscion squamosissimus, dam, population structure, reproductive period.In Brazil the most common direct causes of biodiversity loss in inl...